This invention pertains generally to pulse Doppler tracking systems and particularly to an improved range gate generator for use in such a system to position a range gate around echo signals from a moving target.
As is well known, the round trip time for a radar pulse to travel to and from a target is a function of the range between a radar and the target. If the target is approaching or moving away from the radar, i.e. if a so-called Doppler velocity (or acceleration) is experienced, then the range between the target and the radar continually changes and a concomitant change in the time of arrival of echo signals from the target is experienced. The magnitude of such change in the time of arrival is of particular importance when the radar is a pulse radar installed on a guided missile and a range gating technique is used to reduce the effects of interfering signals, such as those from clutter. Thus, for example, if the Doppler velocity between a guided missile carrying an active seeker and a target were on the order of, say, 2300 meters per second, the range between the two would decrease by approximately 91 meters in each successive interval of approximately 40 milliseconds (a typical interval between successive updates of the contents of a computer). A change in range of such magnitude corresponds to a change in the round trip time in the order of 300 nanoseconds. With a typical width of 160 nanoseconds for the range gate it would be possible for the echo signal from the target to move out of the range gate, resulting in the loss of the echo signal. To avoid loss of the echo signal, the interval between successive updates of the position of the range gate could be shortened or the interval in which the range gate is opened could be lengthened. Neither expedient is, however, desirable for reasons obvious to one of skill in the art. It remains, therefore, to adapt the technique referred to in the art as "velocity aiding" to allow the proper positioning of a range gate in accordance with the Doppler velocity between a target and a guided missile. In carrying out the velocity aiding technique the Doppler velocity is measured or estimated and then the position of the range gate is adjusted in accordance with such velocity so that a desired echo signal may be caused always to fall within a relatively narrow range gate.
Unfortunately, known ways of implementing the velocity aiding technique are, however, not particularly well suited for use where there exists severe restrictions on size and complexity as, for example, in guided missiles. Thus, one known arrangement for implementing velocity aiding involves the use of a tapped delay line wherein discrete delay intervals, each separated by a desired velocity aiding increment, are produced at the taps on such line. The amount of velocity aiding, i.e. the change in the position of the range gate, then is determined by the particular tap selected. The delay line (which is an analog device) and the requisite control circuitry to obtain satisfactory operation under any condition experienced in practice combine to make such an implementation infeasible, by reason of physical size alone, for use in a guided missile.
Another known velocity aiding technique involves the use of two integrators. One of the integrators generates a sawtooth waveform at the pulse repetition interval (PRI) of the radar and the second one of the integrators generates a ramp waveform whose slope depends on the Doppler velocity. The sawtooth waveform and the ramp waveform are applied to a differential amplifier with the result that when the voltage of the former exceeds the voltage of the latter an output is produced by the differential amplifier to trigger a fixed pulsewidth unistable multivibrator. The output of such multivibrator then is the basic waveform for generating the range gate during each PRI. As the slope of the ramp waveform is changed to correspond with different Doppler velocities the instant at which the fixed pulsewidth unistable multivibrator is triggered is similarly changed to effect velocity aiding. As with the implementation using a tapped delay line, the physical size and complexity of the elements making up this implementation militate against its use in a guided missile. In particular, if the change in position of the range gate is to be linearly related to Doppler velocity, the sizes of the capacitors in the integrators are excessive.
Another difficulty with each one of the various known techniques for velocity aiding is that system flexibility may be attained only at the price of complexity. That is to say, if the velocity aiding arrangement is to be usable in a pulse radar whose pulse repetition interval may be changed or if the number of velocity aiding increments (sometimes referred to as "range gate slips") is to be changeable, the complexity of the requisite velocity aiding arrangement is excessively high.